This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Rhesus macaques were immunized with three different strains of single-cycle SIV that express envelope glycoproteins modified to lack structural features that interfere with the development of neutralizing antibodies to determine if enhancing envelope-specific antibody responses might improve the extent of protection afforded by this vaccine approach.